Make sure you have 777 permissions for the ffmpeg.exe file located in your ray folder.

You also have to make sure that you are allowed to run ffmpeg. You can test it out with with a test page that you can find at http://www.boonex.com/trac/ray/wiki/ and look for the GenRayTShooter. It's a file that you run on the server to see if you have the right setup.